The invention relates to a multiband antenna which finds utility particularly in mobile stations.
In the field of portable radio equipment, mobile stations in particular, fabrication of antennas has become more demanding than before. As new frequency bands are introduced, an antenna often has to function in two or more frequency bands. For convenience, the antenna is preferably placed inside the covers of the apparatus. Understandably, however, the radiation characteristics of an internal antenna are somewhat weaker that those of an external antenna. This disadvantage can be reduced using a double-action antenna so that a movable antenna element belonging to the structure can be pulled partly out from the apparatus when necessary, in order to improve the quality of network connection. On the other hand, the increase in the number of radiating structural components means more matching problems.
An antenna with satisfactory characteristics which fits inside a small device is in practice most easily implemented as a planar structure: The antenna comprises a radiating plane and a ground plane parallel therewith. The number of operating bands can be increased to two by dividing the radiating plane into two branches of different lengths, as viewed from the feed point. The structure shown in FIG. 1, disclosed in patent application publication F1991359, represents such a known antenna. It comprises a ground plane GND, radiating planar element 120, feed conductor 103 of the latter, and a short-circuit conductor 104 which connects the radiating plane to the ground plane. So, considering this construction, the antenna is a so-called planar inverted F antenna (PIFA). The planar element 120 includes a slot 130 starting from the edge of the planar element and ending in the inner area of the planar element after a U-shaped bend. The feed point F of the planar element is near the U bend of the slot 130. Viewing from the feed point, the first branch B1 of the planar element which curves inside the U bend is electrically clearly longer than the second branch B2 at the other side of the planar element. Thus the PIFA has got two separate resonance frequencies and respective operating bands. The antenna structure of FIG. 1 further comprises a movable whip element 111, at the lower end of which there is a conductive connecting part 112. When the whip is in its lower position, it has no significant coupling with the PIFA parts. When the whip is in its upper position, as shown in FIG. 1, the connecting part 112 is in galvanic contact with the radiating planar element 120 near its feed point F. So, the whip element is fed via the planar element. The dimensions of the whip element are such that it resonates in the lower operating frequency band of the PIFA part, where it improves the electrical performance of the antenna. The whole structure thus has two operating bands. If one would attempt to provide a third operating band using e.g. a whip element, there would be matching problems and the antenna characteristics at least in one operating band would be unsatisfactory. A third operating band could be provided by a separate radiating element and antenna port but this would be space-consuming and require additional costs.
From patent application publication F1990006 it is known to use a slot radiator to provide a second operating band for an antenna. The publication discloses a PIFA-type antenna where the radiating planar element includes a slot extending from the edge to the middle region. The feed and short-circuit points of the antenna are close to the open end of the slot. The dimensions of the slot are such that it resonates in the upper one of the intended operating bands. The conductive planar element naturally resonates in the other, lower operating band. This antenna structure, too, has only got two operating bands. Moreover, to add a whip element in the known manner would result in matching problems in the operating bands of the PIFA.
An object of the invention is to realize in a new, more advantageous manner a mobile station antenna which has at least three useful operating bands. An antenna structure according to the invention is characterized by that which is specified in the independent claim 1. Some advantageous embodiments of the invention are presented in the dependent claims.
The basic idea of the invention is as follows: The antenna structure includes a PIFA-type antenna placed inside the covers of a mobile station, which antenna has a slot radiator formed by a first slot starting from near the feed and short-circuit points of the planar element. In addition, the planar element has a second slot so that, viewed from the feed point, there are formed two radiating conductive branches of different lengths. Each of the three radiators has a separate operating band of its own. The structure further includes a whip element movable in relation to the PIFA part. When pushed in, the whip element has no significant coupling with the antenna feed or PIFA parts. When pulled out, the lower end of the whip element is galvanically coupled to the shorter branch of the planar element. For this coupling, the shorter branch has a relatively narrow side branch. The dimensions are chosen such that the whip enhances the operation of the antenna especially in the lowest operating band. The influence of the pulled-out whip on the location of the uppermost operating band is compensated for by a third slot made in the planar element, with which third slot at the same time the shorter branch of the planar element is shaped.
An advantage of the invention is that the whip element enhancing the operation of the antenna can be coupled without any special matching arrangements. Matching is done by shaping the conductive patterns that exist in the planar element in any case. From this it follows that a structure according to the invention is relatively simple and economical to fabricate. Another advantage of the invention is that it provides a useful three-band planar antenna without a whip element, too.